1. Field of the Invention
The present invention relates to a carrier for developing an electrostatic latent image used for an electrophotographic method and an electrostatic recording method, and a developer and an image forming apparatus which use the carrier for developing an electrostatic latent image.
2. Description of the Related Art
In recent years, technologies of copiers or printers using an electrophotographic system has been rapidly expanding from monochrome to full color, and there is a tendency that a full-color market is expanding. In color image formation by full-color electrophotography, an electrostatic latent image is formed on an electrostatic latent image bearing member; a toner image is formed by developing this electrostatic latent image with charged color toners of three (3) colors of yellow, magenta and cyan or with color toners of the above 3 colors with an addition of black; and then this toner image is transferred and fixed on a recording medium.
In the image formation by such a full-color electrophotographic system, in order to obtain a vivid full-color image having superior color reproducibility, it is necessary to maintain a toner amount on the electrostatic latent image bearing member closely to the electrostatic latent image. This is because fluctuation of the toner amount on the electrostatic latent image bearing member changes an image density or a color tone of an image on a recording medium.
As a cause of fluctuation of the toner amount on the electrostatic latent image bearing member, although there is a factor of fluctuation in a toner charge amount, a so-called hysteresis (ghost phenomenon) is reported that, in a hybrid development system in particular, a difference in the toner amount on a toner bearing member appears as a density difference on an image in subsequent development (see Japanese Patent Application Laid-Open (JP-A) No. 2007-25693).
As a method for resolving the hysteresis in the hybrid development system, it is effective to remove once a residual toner on the toner bearing member after development of the electrostatic latent image on the electrostatic latent image bearing member and to supply a new toner on a surface of the toner bearing member so as to eliminate a difference in the toner amount on the toner bearing member as described above. For example, a method for resolving the hysteresis by scraping a residual toner on the toner bearing member after development and before toner resupply by a scraper or a toner recovery roller (see Japanese Patent (JP-B) No. 3356948, JP-A No. 2005-157002, JP-A No. 11-231652). Also, there is proposed a method for resolving the hysteresis by collecting the residual toner on the toner bearing member by means of a potential difference in a magnetic roller during intervals of copying or between paper and by stabilizing the toner amount on the toner bearing member (see JP-A No. 07-72733). Also, as a countermeasure of the hysteresis using a magnetic brush, there is proposed a method for resolving the hysteresis by widely setting a half-width area of a magnetic flux density of the magnetic roller for collecting the toner on the developing roller and stabilizing the supply (see JP-A No. 07-128983). Further, there is proposed a method for resolving the hysteresis by maintaining the constant toner amount on the toner bearing member without being affected by a history of an immediately preceding image. That is, a non-spherical carrier is used as a carrier for a two-component developer; charge is injected to the carrier at a tip of the magnetic brush; thereby, substantial spacing between the developer bearing member and the toner bearing member is narrowed, and a toner amount supplied at once to the toner bearing member is increased; and the toner is supplied up to a toner saturated amount on the toner bearing member (see JP-A No. 07-92813).
The above-described hysteresis has been considered as a problem specific to the hybrid development system, but it is reported that use of the developer over a long period of time in a two-component developing system causes a hysteresis with decreased developing capacity and decreased image density (see JP-A No. 11-65247).
The hysteresis in the two-component developing system is caused by unsuccessful denuding of the two-component developer. The developer is denuded as follows. That is, an odd number of magnets are disposed in a developing sleeve; a pair of magnets of the same polarity is disposed at a location below an axis of rotation of the developing sleeve to create a peeling region where a magnetic force is almost zero; the developer after development is allowed to fall freely using the gravity in the region.
However, a counter charge is generated in the carrier when the toner is consumed in an immediately preceding image, which causes an image force between the carrier and the developer bearing member, and as a result, the developer is not successfully released in the peeling region. Thus, the developer with a decreased toner concentration due to the toner consumption is conveyed again to a developing region, resulting in degraded developing capacity. That is, the concentration is normal for one round of the sleeve, but the concentration decreases for the second round or after.
As a method for resolving the hysteresis in the two-component developing system, for example, there is proposed a method to dispose a pumping roller including a magnet inside near the peeling region on the developing sleeve and to peel the developer after development by means of the magnetic force (see JP-A No. 11-65247). The peeled developer is lifted by another pumping roller and then conveyed to a developer stirring chamber including a screw, where the toner concentration is readjusted and the toner is charged.
However, since there is an effect of the hysteresis in long-term continuous use despite the above proposals, there are problems that a stable toner amount cannot be supplied to the electrostatic latent image bearing member and that a vivid image having superior color reproducibility cannot be obtained. Also, depending on the above proposals, there is a problem specific to the two-component developing system that a change in carrier resistance due to accumulation of the spent toner is large and that a decrease in charging property of the carrier is large. Further, in order to solve the above problems, it is necessary to maintain various properties of the carrier that it does not cause apparatus contamination due to background smear, toner scattering and so on. Accordingly, it is strongly desired at present that these problems are solved simultaneously.